This invention relates to a process for the conversion of coal to a clean fuel, that is, a fuel which is substantially free of the mineral components normally found in coal.
In particular, the invention relates to a method for separating undissolved solids from a coal liquefaction product wherein the coal liquefaction product is admixed with a hydrogenated coal oil solvent, having been hydrogenated to remove at least about 80% of the asphaltenes and with a light aromatic solvent. The primary product of the coal liquefaction process is a mixture of liquid and undissolved solids. A portion of the undissolved solids appears as extremely finely divided particles of the order of ten microns or less in size. These particles are rich in mineral matter normally found in all coals. Combustion of fuel containing these particles forms ash.
Complete separation of such finely divided particles from the liquid in which they are suspended cannot be accomplished by the usual mechanical separation techniques at ordinary temperatures, which techniques include filtration, centrifugation, and settling, because of the extremely fine state of subdivision of the solid particles and because of the high viscosity of the liquid. Separation is improved by operation at elevated temperatures due to a rapid decrease in liquid viscosity, as well as an increase in the density differential between liquid and solid. Even at these elevated temperatures and reduced viscosities, the conventional separation techniques may be only partially effective.